Error message on cpdef variables (GH-3963)

Allowing these gives people the false impression that they do something meaningful.

Closes https://github.com/cython/cython/issues/887
Closes https://github.com/cython/cython/issues/3959

Cython/Compiler/Nodes.py

@@ -1430,8 +1430,7 @@ class CVarDefNode(StatNode):
                     self.entry.create_wrapper = True
             else:
                 if self.overridable:
-                    warning(self.pos, "cpdef variables will not be supported in Cython 3; "
-                            "currently they are no different from cdef variables", 2)
+                    error(self.pos, "Variables cannot be declared with 'cpdef'. Use 'cdef' instead.")
                 if self.directive_locals:
                     error(self.pos, "Decorators can only be followed by functions")
                 self.entry = dest_scope.declare_var(

tests/errors/cpdef_vars.pyx

-# tag: warnings
+# mode: error
 
 cpdef str a = "123"
 cpdef b = 2
@@ -16,9 +16,9 @@ def func():
     return d
 
 
-_WARNINGS = """
-3:6: cpdef variables will not be supported in Cython 3; currently they are no different from cdef variables
-4:6: cpdef variables will not be supported in Cython 3; currently they are no different from cdef variables
-7:10: cpdef variables will not be supported in Cython 3; currently they are no different from cdef variables
-15:10: cpdef variables will not be supported in Cython 3; currently they are no different from cdef variables
+_ERRORS = """
+3:6: Variables cannot be declared with 'cpdef'. Use 'cdef' instead.
+4:6: Variables cannot be declared with 'cpdef'. Use 'cdef' instead.
+7:10: Variables cannot be declared with 'cpdef'. Use 'cdef' instead.
+15:10: Variables cannot be declared with 'cpdef'. Use 'cdef' instead.
 """